1. Field of the Invention
The present invention relates to a developing toner for electrostatic latent images, an imaging forming method and an image forming apparatus.
2. Description of the Related Art
Electrophotographic image forming apparatus such as copying machines, printers and facsimiles are adopted to form electrostatic latent images by various image forming processes on photoreceptors, then develop the electrostatic latent images by a toner supplied from a developing device into toner images, transfer the toner images to a recording material and then fix them by heating and pressing rollers thereby obtaining images. As a photoreceptor, an organic photoreceptor having a photoconducting layer containing an organic polymeric compound and a conductive material has been used generally in view of high safety in the manufacturing process, high safety to environments, and reduced material cost.
Since the electrophotographic image forming apparatus are easy for operation and maintenance and can form images having good picture quality at a relatively high speed, they are apparatus indispensable in various cases. Recently, reduction of the size has been progressed further with an aim of space saving. Correspondingly, reduction of the size has been demanded further for the constituent devises inside the image forming apparatus, for example, a light sensitive body, plural devices arranged at the periphery thereof and a toner containing vessel.
For the image forming apparatus, in addition to the size reduction, it is demanded to increase the image forming speed and increase the number of sheets capable of forming images per shot of the toner to be filled. While increase in the number of sheets capable of forming images can be attained by increasing the capacity of the toner containing vessel, since there is a conflicting demand of reducing the size of the apparatus on the other hand, it is not possible to simply increase the capacity. Accordingly, it is demanded to contain a more amount of the toner in the size-reduced toner containing vessel than usual, or to decrease the amount of the toner consumed per cycle of image formation. For this purpose, it may be considered to reduce the size of toner particles while enhancing the hiding power of the toner on a recording material. However, as the size of the diameter of toner particles is decreased, the triboelectric charging amount of individual toners is lowered and the toner scatters from a developing machine upon developing electrostatic latent images to contaminate the inside of the image forming apparatus. Furthermore, as the triboelectrical charging amount of the toner lowers, since the Van der Walls force between toner particles increases to cause agglomeration of toners, no uniform toner images can be formed on electrostatic latent images to cause image failure. Furthermore, background fogging of deposition of the toner to a portion other than the portion for electrostatic latent images also occurs remarkably to extremely increase the toner consumption amount. Furthermore, upon transfer of toner images on the photoreceptor, blotting of character images (phenomenon of toner scattering at the periphery of character images), trailing (phenomenon of toner deposition, like trailing, to the rear end of character images) etc. also occur.
Furthermore, in a case of forming images by using a toner, oxides for improving the toner flowability such as silicon dioxide, titanium oxide or aluminum oxide is added to the toner. Furthermore, in a case of use as a magnetic toner, an inorganic magnetic material is used together. Such oxides and inorganic magnetic materials have a property of grinding the photoconducting layer of an organic photoreceptor drum. Accordingly, the photoconducting layer is gradually ground into a thin layer in the process of repeating image formation. As a result, the residual potential in the photoconducting layer increases to often cause failure of picture quality such as background fogging, increase of the line width for fine line images and, in addition, this shortens the working life of the organic photoreceptor drum. In view of the situations described above, it has been proposed to use a metal oxide power with a weight average grain size of 0.6 to 5 μm, a number average grain size of 0.5 to 4 μm and a weight average grain size/number average grain size ratio of 1.0 to 2.4 as the toner fluidizing agent instead of the oxides described above (for example refer to Japanese Patent Publication JP-A 6-3854(1994)). While the metal oxide powder is effective for ensuring the toner flowability, it shows high grinding property and has the same defect of shortening the working life of an organic photoreceptor drum as that of existent oxides or inorganic magnetic materials.
On the other hand, as a binder resin which is a matrix for carrying a colorant, a polyester resin is used generally in the toner. By the use of the polyester resin, the toner can be fixed at a relatively lower temperature to a recording material. Furthermore, since the polyester resin has such a triboelectric chargeability as required for the toner, it has an advantage capable of preventing filming in which a toner is deposited not to the recording material but to the photoreceptor. However, the toner using the polyester resin as the binder resin involves a drawback of causing a so-cold offset phenomenon in which the toner is deposited to heating and pressing rolls in a case of fixing toner images on the recording material by the heating and pressing rolls. Then, waxes are added as an anti-offsetting agent. The waxes include, for example, non-polar low molecular weight polyolefin, teflon (registered trademark) resin and paraffin. However, since the anti-offsetting agents described above have less compatibility with the polyester resin, they can not prevent the occurrence of the offset phenomenon sufficiently. In addition, the anti-offsetting agent is present as it is in the toner and a portion thereof is fused to a photoreceptor or deposited to a carrier used together with the toner, making the toner chargeability not uniform and causing the phenomenon like filming.
Furthermore, for improving the hiding power of the toner, it is extremely important to control the grain size and the deposition amount of the wax deposited on the surface of toner particles. In a case where a great amount of wax particles are present on the surface of the toner particles, they make it difficult to provide triboelectric charges to the toner, thus makes it difficult to obtain a desired triboelectriccally charged amount of the toner, and increase the toner particles with less charged amount and, thus, tending to cause disadvantages such as toner scattering or background fogging. Furthermore, in a case where the deposition amount of the wax particles on the surface of the toner particles is excessively small, this naturally tends to cause the offset phenomenon and deteriorates the fixing property of the toner to the recording material.
In a developing toner for electrostatic latent images containing a polyester resin as a binder resin, use of an oxidized type polyolefin with an acid value of 1 or more in which an unsaturated carboxylic aid is graft polymerized to a polyolefin resin as an anti-offsetting agent has been proposed (for example refer to Japanese Unexamined Patent Publication JP-A 59-129863(1984)). In the invention disclosed in JP-A 59-129863, since the oxidized type polyolefin having a polarity is added to a polyester resin having polarity by nature, compatibility between both of them is enhanced and fusion of the oxidized type polyolefin to a photoreceptor and deposition thereof to a carrier can be avoided reliably. Furthermore, the oxidized type polyolefin is effective also for the prevention of the offset phenomenon. However, since a general combination of a polyester resin and an oxidized type polyolefin shows excessively high compatibility and the oxidized polyolefin is dispersed excessively as fine particles, high lubricity inherent to the oxidized type polyolefin can not be provided sufficiently. As a result, releasability between the heating and pressing rollers and paper as the recording material is insufficient. Particularly, in a case where solid black images are present at the top end of the images, this results in a problem of causing paper jam in the image forming apparatus. Furthermore, since the toner is poor in slidability on the surface of solid black images after fixing, so-called fixing property to scratching, it results in a problem of back transfer or back contamination in which recording materials are rubbed to each other and the toner on the surface of the recording material is deposited to the surface opposite to the image forming surface of the recording material upon stacking several sheets of recording materials on which images are formed and manually turning back them for inspection.